This invention relates to a gas turbine system in which a combustor exhaust pipe of a gas turbine and gas turbine blades are steam cooled.
A conventional gas turbine system is described with reference to FIG. 4.
As shown in FIG. 4, a conventional gas turbine system which uses steam in cooling the blades of a gas turbine 4 is configured as follows: Cooling steam is bled from an outlet of a high pressure turbine 7, and cools the blades of the gas turbine 4. The steam is then recovered into an inlet of an intermediate pressure turbine 8. A tail pipe of a combustor 6 is not steam cooled, but is air cooled with compressed air.
The other subsystems of the gas turbine system constitute the same system diagram as for a gas turbine combined cycle plant in general use. The numeral 01 denotes a gas turbine, 02 an exhaust heat recovery boiler, 7 a high pressure turbine, 8 an intermediate pressure turbine, and 9 a low pressure turbine. The air is sucked into a compressor 5, and compressed there to a predetermined pressure. Then, fuel adjusted to have a predetermined temperature at an inlet of a turbine 4 and air pressurized by the compressor 5 are mixed and burnt in the combustor 6.
An exhaust pipe portion of the combustor 6 is of the type to be cooled with discharge air from the compressor 5. That is, at the inlet of the turbine 4, a high temperature, high pressure combustion gas generated by the combustor 6 is expanded to produce output, which causes a generator 18 to generate power. Exhaust gas after completion of work is supplied to the exhaust heat recovery boiler 02 through an exhaust gas duct 10.
A turbine stationary blade and a turbine moving blade of the turbine 4 are cooled with steam fed from an outlet of the high pressure turbine 7 through a blade cooling steam supply piping 11. Heated steam after cooling is recovered into an inlet of the intermediate pressure turbine 8 via a blade cooling steam recovery piping 12.
In the exhaust heat recovery boiler 02, on the other hand, superheated steam is produced by a low pressure drum 1, an intermediate pressure drum 2, and a high pressure drum 3. Steam generated by the high pressure drum 3 is guided to the high pressure turbine 7 through a high pressure steam piping 13, and expanded in the high pressure turbine 7 to generate output.
Output steam from the high pressure turbine 7 is branched into a portion which is guided to the turbine stationary blade and moving blade of the gas turbine 01 by the blade cooling steam supply piping 11, and a portion which is guided to a reheater of the exhaust heat recovery boiler 02.
Steam produced by the intermediate pressure drum 2 is sent through an intermediate pressure steam piping 14, and mixed with the outlet steam from the high pressure turbine 7 at the inlet of the reheater. Then, the mixed steam is introduced into the reheater, where it is heated. The heated steam is mixed with the blade cooling steam guided by the blade cooling steam recovery piping 12, and fed to the inlet of the intermediate pressure turbine 8.
Then, this steam is expanded by the intermediate pressure turbine 8 to generate output. Then, it is mixed with superheated steam which has been produced by the low pressure drum 1 and supplied via a low pressure steam piping 15. The mixture is fed to an inlet of the low pressure turbine 9.
The steam supplied to the inlet of the low pressure turbine 9 is expanded by the low pressure turbine 9 to generate output to a generator 19. Then, steam is condensed by a condenser 20, and pressurized by a pressure pump 21 to a predetermined pressure. Then, the condensate is fed to the exhaust heat recovery boiler 02 through a feed water piping 22.
As stated earlier, according to the conventional steam cooled gas turbine system, output steam from the high pressure turbine 7 was used only for blade cooling. The site of its recovery was the inlet of the intermediate pressure turbine 8. The cooling of the exhaust pipe of the combustor 6 was performed not with steam, but with outlet air from the compressor 5.
With such a gas turbine system, it has been customary practice to elevate the inlet temperature of the turbine 4 in order to achieve a further improvement in thermal efficiency. If the inlet temperature of the turbine 4 is raised, however, cooling air for the exhaust pipe of the combustor 6 will be needed in a large amount. Thus, combustion air based on the same supply source will decrease. As a result, the equivalence ratio of fuel to the combustion air lowers, causing a possibility for unstable combustion.
Furthermore, the necessity for further raising the combustion temperature by the increase in the amount of cooling air for the combustor 6 will lead to an increase in the amount of NO.sub.x generated. Also, the temperature distribution in the radial direction of the outlet of the turbine 4 will enlarge, reducing the efficiency of the turbine 4.